Field of the Invention
The invention concerns a magnetic resonance imaging method of the type wherein a number of repetitions of a magnetic resonance measurement sequence and a number of repetitions of a navigator magnetic resonance measurement sequence are executed in a interleaved manner, wherein the phase of the magnetic resonance data that are obtained based on the magnetic resonance measurement sequence is changed on the basis of the navigator magnetic resonance data. The invention also concerns a magnetic resonance apparatus, and a storage medium encoded with programming instructions, for implementing such a method.
Description of the Prior Art
Magnetic resonance (MR) imaging is a technology for creating MR images that map an examination object. Typically the examination object is positioned in a constant magnetic field that is designed as statically and as homogeneously as possible, which has strength of between 0.5 Tesla and 5 Tesla, for example. The constant magnetic field aligns the nuclear magnetization of the examination object; in particular a polarization of the nuclear spin magnetization takes place in the direction of the constant magnetic field.
Radio-frequency (RF) pulses can then be radiated in order to deflect the nuclear magnetization from its rest position in the direction of the constant magnetic field, i.e. in order to excite the nuclear magnetization. The subsequent relaxation of the nuclear magnetization creates RF signals, so-called echoes. Within the framework of gradient echo MR imaging or echo planar MR imaging (EPI), so-called gradient echoes are explicitly created by gradient pulses being applied for rephasing and dephasing the nuclear magnetization.
Gradient pulses are applied for spatially encoding the MR data. The gradient pulses create gradient magnetic fields (gradient fields) that are superimposed on the constant magnetic field.
The MR data can be measured during a readout phase. Frequently the acquired MR data are referred to as raw data. The MR data can be processed in order to reconstruct the MR image of the examination object. For example, the measured MR data are typically digitized and are initially present as data entries in the spatial frequency space (k-space). On the basis of a Fourier transformation, it is then possible to transform the MR data into the image space in order to create the MR image.
Within EPI, it can be possible for the MR data to exhibit artifacts that adversely affect the imaging of the examination object. The gradient pulse train that is typically applied within the framework of the EPI has a number of gradient pulses of different polarity in a sequential order. Depending on polarity, the gradient echoes will sometimes be referred to as even or odd. Because of the alternating polarity of the gradient pulses of the gradient pulse train, MR data for different rows of k-space are measured in an alternating direction. This means, for example, that if the data entry trajectory in k-space is row-by-row, the MR data are entered from left to right for a first row and for a second row, in k-space adjacent to the first row, the data are entered from right to left.
In EPI, errors of the phase (phase errors) of the MR data can cause artifacts. This can result in shifts of the phase of the MR data for rows in k-space with a different entry direction, as described above. This can occur, for example, because of imprecise timing during application of the gradient pulses and/or during digitization within the framework of the measurement of the MR data and/or because of eddy current effects. Such an offset of the phase of the MR data in adjacent rows of k-space can lead to so-called N/2 ghosting artifacts. Such an N/2 ghosting artifact can occur in the MR image as “ghost” mapping of the examination object and typically has a lower intensity than the actual mapping of the examination object, and may also be shifted in the positive and/or negative direction in relation to the actual mapping.
A further source for errors in the phase of the MR data can be a temporal dependency of the amplitude and/or of the direction of the constant magnetic field (drift). A typical cause for a drift of the constant magnetic field is, for example, heating or mechanical vibration of the hardware of the MR system while the EPI is being carried out. Typical artifacts that can occur as a result of the temporal drift of the constant magnetic field include loss of contrast, shifting of the object in the reconstructed image in the phase encoding direction, for example.
Techniques for reduction of such artifacts in EPI are known. For example a technique is known from U.S. Pat. No. 6,043,651 for correcting the phase on the basis of navigator MR data. Through the reduction of the artifacts it can be achieved that the quality of the MR images is improved and thereby that there is a greater information content in the MR images. The imaging will be improved. In the medical field this enables more precise diagnoses to be made or errors to be avoided during diagnosis. The entire content of the disclosure of U.S. Pat. No. 6,043,651 relating to the change of phase of MR data for reduction of artifacts is incorporated herein by reference. U.S. Pat. No. 6,043,651 describes how a phase offset between even and odd gradient echoes, i.e. gradient echoes with opposing readout direction, can be reduced by correlation of the MR data on the basis of the navigator MR data. A retrospective alignment of the odd and even gradient echoes can be achieved to compensate for a phase offset.
Furthermore a technique is known from US 2012/0249138 A1 for reducing artifacts resulting from a drift of the constant magnetic field (dynamic off-resonance in k-space, DORK). The entire content of the disclosure of US 2012/0249138 A1 relating to the change of phase of MR data for reduction of artifacts is incorporated herein by reference. US 2012/0249138 A1 describes how the constant magnetic field drift can be determined by comparing the phase evolution of echoes with identical polarities of the readout gradient pulses between consecutively measured MR data. Typically, within the framework of the DORK technology, such a computation will be carried out averaged over an entire imaging region.
For specific forms of EPI it can be difficult to apply such known techniques for reduction of artifacts. For example, within the framework of simultaneous multi-slice (SMS) EPI, in which the nuclear magnetization is excited in a number of slices of the examination object by radiating a suitable RF excitation pulse, and gradient echoes of the nuclear magnetization excited by the RF excitation pulses are created in parallel in time, it can be impossible or only possible to a restricted extent to have direct access to the navigator MR data, in order to make the corresponding corrections as described above. Such techniques are sometimes also referred to as slice multiplexing techniques.
It can be necessary, for example in reference implementations, to obtain navigator MR data on the basis of gradient pulses, which are subsequently applied directly within the framework of the simultaneous multi-slice imaging to a corresponding RF excitation pulse. A phase encoding of the gradient echoes of the navigator MR data takes place, so that a separation based on SMS technology is possible. This enables the echo time TE between excitation and creation of the echo for gradient echoes of the SMS EPI to be extended, which typically is disadvantageous for the quality of the MR imaging. Sometimes the temporal resolution can also be restricted with such approaches, since only half of the correction information can be obtained per navigator for example. Also in such a case the quality of the MR imaging can be restricted. See U.S. patent application Ser. No. 14/868,529, for example.
Other reference implementations are based on a one-off preceding navigator MR measurement sequence. Such techniques cannot detect, or can only poorly detect, time dependencies of the phase errors during the measurement, so that imprecisions result. The drift of the constant magnetic field cannot be mapped or can only be mapped to a restricted extent. See, for example, SETSOMPOP K. et al., “Improving diffusion MRI using simultaneous multi-slice echo planar imaging” in NeuroImage 63 (2012) 569-580.